1. Field of the Invention
The present invention relates to a chrominance signal processing circuit that processes a chrominance signal by using a semiconductor field memory for use in a field/frame CNR (chroma noise reducer).
2. Description of the Related Art
In a field/frame CNR that reduces random noise contained in a chrominance signal or the like, an analog chrominance signal is converted to a digital signal. Then, the digital signal is written in a field/frame memory and noise is reduced by effectively utilizing an interfield/interframe correlation of a video signal.
When the analog chrominance signal is converted in the form of analog to digital signal, a sampling frequency for the A/D (analog-to-digital) conversion is 4 fsc (fsc is a color subcarrier frequency) that is the same as the sampling frequency used when a luminance signal, for example, is subjected to a sampling processing. Therefore, when such chrominance signal is written in the above memory, if the number of samples of chroma signal on one line is 910 samples, the number of memory lines is 250 lines and a data gradation is formed of 8 bits, then a memory having a large capacity of about 1.8 Mbits is required and a chrominance signal processing circuit becomes expensive.
To remove the aforesaid drawback, it has been proposed that the amount of data be reduced to 1/2 by writing a chroma signal in a memory at every other line in the vertical direction and that the number of bits of the memory be reduced by forming a data gradation by 4 bits. These previously-proposed methods cannot overcome the above defects. That is, if the number of lines written in the memory is reduced, then a so-called dither occurs in an oblique portion of a picture when a reproduced signal is reproduced on a picture screen of a television monitor. Further, if the data gradation is reduced, then there occur various problems that a color cannot be reproduced with high definition. The problems that the present invention intends to solve are as follows. That is, when the chroma signal is processed by using the memory, if the number of lines written in the memory is reduced and the data gradation is reduced in order to reduce the number of bits of the memory, there are then various problems that the definition of a reproduced chroma signal is deteriorated and that a distortion occurs.